there are 2 options i guess,you will need Pokemon Ruby,Sapphire or Emerald and find Deoxys in birth island(if you solve some kind o puzzle)and migrate it.
2.Get the Diamond or Pearl event in GameStop and the Action Replay,then talk to a guy in any PokeMart,he will give you a Deoxys then trade it.
The REAL thing to do...,well i don't know but go here.
http://www.serebii.net/pokedex-dp/386.shtml
You can't catch Deoxys in soul silver
If you have action replay you can get any Pokemon you want. This feature works only in Pokemon Heartgold , Soulsilver and Platinum.
First you have to catch Mewtwo and then head to Mt Moon at midnight on Monday. Somewhere deep in the cave is were the deoxys are.
Sorry to rain on your parade, you can't get it on soulsiver. sorry! :) -pokeman
it does not exist in soulsilver outside of hacks or trades.
you can't
You can't catch Deoxys in soul silver
Well not you... me?
They change deoxys form
You have to trade a Deoxys from another Pokemon game to get it on your copy of SoulSilver.
#386
386
Deoxys is not a wild Pokemon in SoulSilver. Deoxys has to be traded into the game in order for you to get it. Deoxys can also be received by event, and one recently took place in Japan for Deoxys.
You can't catch Deoxys in the wild or a special place. But if you got it in Pokemon FireRed, LeafGreen, Sapphire, Ruby, or Emerald you can migrate Deoxys in the Pal Park in the Kanto region in Fuchsia City.You cannot obtain Deoxys legitimately in Pokémon SoulSilver. You must take advantage of an Event in Pokémon Emerald, FireRed, or LeafGreen to go to Birth Island using an Aurora Ticket, and catch it there, then use the Pal Park to transfer it to SoulSilver. Alternatively, you can cheat.
You cannot find Deoxys in HeartGold, or SoulSilver. You can only get Deoxys through trading or at a Pokemon Event.
You cannot find a wild Deoxys in in Pokemon Soul Silver, or HeartGold. Deoxys can only be obtained by trading with someone or by getting one at special distributions.
Sadly, there is no way to get Doexys in Pokemon SoulSilver besides trading.